In recent years, increase in transmission speed has been discussed for optical transmission systems that transmit wavelength division multiplexing (WDM) light. As the transmission speed is increased, however, the transmission characteristics are expected to deteriorate. A digital coherent receiving scheme is one of techniques useful for keeping the good transmission characteristics even if the transmission speed is increased. The digital coherent receiving scheme is a technique of high-speed digital signal processing for demodulating signal light separated from WDM light.
An optical reception apparatus that employs the digital coherent receiving scheme generates mixed light of local oscillation light and signal light separated from WDM light, coverts the generated mixed light into electric signals for demodulating the signal light, and amplifies each power of the electric signals within a predetermined tolerance level using an amplifier. Thereafter, the optical reception apparatus that employs the digital coherent receiving scheme demodulate the signal light by applying digital signal processing to the electric signals output from the amplifier.
This technique is disclosed in Japanese Laid-open Patent Publication No. 2013-38745, for example.
However, the above-mentioned conventional technique has a problem of the lowering of the demodulation accuracy of the signal light when the power of the signal light transitionally fluctuates due to change in the number of channels of the wavelength division multiplexing light.
Specifically, when the power of the signal light transitionally fluctuates due to the change in the number of channels of the wavelength division multiplexing light, each power of the electric signals input into the amplification section in the optical reception apparatus is not within the predetermined tolerance level. As a result, the characteristics of electric signals output from the amplifier deteriorate. For example, channel removal in the wavelength division multiplexing light causes the power of the signal light to transitionally rise, so that each power of the electric signals input into the amplification section exceeds the upper limit value in the tolerance level, and the amplifier outputs an electric signals with the deteriorated waveform.
In such a case, the conventional optical reception apparatus applies digital signal processing to the electric signals with the deteriorated waveform. For this reason, the demodulation accuracy of the signal light may be lowered.
Moreover, for example, channel addition to wavelength division multiplexing light causes the power of the signal light to transitionally drop, so that each power of the electric signals input into the amplification section is below the a lower limit value in the tolerance level, which results in the deteriorated signal noise ratio (SN) ratio of electric signals output from the amplifier. In such a case, the conventional optical reception apparatus applies digital signal processing to the electric signals having the deteriorated SN ratio, an, so that the demodulation accuracy of the signal light may be deteriorate.
In one aspect, an object is to provide an optical transmission system, an optical reception apparatus, a management apparatus, and a signal adjustment method which are capable of moderating the lowering of the demodulation accuracy of signal light even when the power of the signal light transitionally fluctuates due to the change in the number of channels of wavelength division multiplexing light.